Introduction Behaviour change trials are increasingly important in public health. Although there has been longstanding awareness of pre-test sensitisation and the Hawthorne effect, the implications of participant reactivity in behaviour change trials are largely unstudied. The aim here is to explore the mechanisms by which biases stemming from the unintended consequences of research participation may be introduced in trials.
Methods Data from systematic reviews, dedicated methodological experimental studies and qualitative studies nested within behaviour change trials will be presented.
Results Study findings indicate: (1) the existence of previously unrecognised recruitment effects on behaviour; (2), that assessment reactivity is a widespread form of contamination in behaviour change trials; (3) that randomisation itself may induce behaviour change; and (4) that these types of effects are highly unlikely to be additive to the effect of behavioural interventions, thus leading to biased estimates of effectiveness.
Conclusions The scale of the problem, the implications for reviews of existing trials, and the reasons for the lack of development of study in this crucial area for behaviour change trials are discussed. Hypotheses and study designs are proposed to guide new research which seeks to quantify problems with existing practice in the design and conduct of trials.